Fuel cells had been proposed as a power source for many applications. So-called PEM (proton exchange membrane) fuel cells a.k.a. SPE (solid polymer electrolyte) fuel cells! potentially have high energy and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM/SPE fuel cells are well known in the art, and include a "membrane electrode assembly" (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. The channels/openings for the reactants are often referred to as "flow channels". A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
PEM fuel cells are typically H.sub.2 --O.sub.2 fuel cells wherein hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode oxidant. The oxygen can either be in a pure form (i.e., O.sub.2), or air (i.e., O.sub.2 admixed with N.sub.2). The solid polymer membranes are typically made from ion exchange resins such as perfluorinated sulfonic acid. One such resin is NAFION.TM. sold by E. I. DuPont deNemours & Co. Such membranes are well known in the art and are described in U.S. Pat. Nos. 5,272,017 and 3,134,697, and in Journal of Power Sources, Volume 29 (1990), pages 367-387, inter alia. The anode and cathode themselves typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles and proton conductive resin intermingled with the catalytic and carbon particles. One such membrane electrode assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention.
PEM fuel cell performance can degrade for a number of reasons such as carbon monoxide poisoning of the anode catalyst, which can occur when the CO-concentration in the H.sub.2 feed stream exceeds about 20 PPM. It is desirable to have an advance warning of any such impending condition to prevent damage to the stack and trigger early corrective action.